A fresh session and one that sat unused for months look identical from the platform's side: no conversations, no history, no incoming messages. And then an account like that suddenly starts writing to dozens of strangers in a row. That is the most recognisable spam pattern there is — far more obvious than anything in the message text.
Warm-up addresses it literally. Before an account touches your list, it exchanges messages with your other accounts following everyday scripts. It ends up with conversations, sent and received messages, and natural intervals between actions. The campaign then starts from something rather than nothing.
The second effect is less obvious and just as useful: a warm-up is a free dress rehearsal. While the accounts talk to each other, the ones with a dead proxy, an unreadable session or an existing restriction drop out. You see them in the table before the campaign rather than halfway through it, when half your messages are already failing.
There is also a hard dependency: without the "Warmed-up account" flag the Checker will not run at all — username verification needs warmed accounts. Warm-up is not an optional extra beside the process, it sits inside the chain of accounts, warm-up, list, campaign.